Corrosion of two similar austenitic stainless steels when welded

Abstract

Two similar grades of austenitic stainless steel (317 L and 317 LM) were welded and their corrosion behaviour noted potentiostatically. It was found that, because of its higher Mo content, 317LM exhibited less active current densities and passive ranges than 317L when the heat affected zone (HAZ) microstructures were examined. This finding held good for a range of δ–ferrite contents representing the whole width of the HAZ. In the weld bead, however, the behaviour of both materials was essentially similar. Electron optical studies were performed on polarized samples and it was found that in both HAZ and weld bead regions there was a distinct Mo denudation zone at the δ/γ boundary. It was found that in HAZ material the reduction of Mo was associated with the production of very small particles on the δ/γ grain boundary, believed to be metallic (mainly Mo) carbides. From these findings it was concluded that it is unwise to discuss the corrosion resistance of welded austenitic stainless steels without detailing the method of formation of the δ-phase. To aid this, solid–phase transformation δ is termed δs and liquid–phase formed δ,δL.MST/115